Recording materials such as, for example, recording sheets, are used to provide an image and are well-known today. Thermal recording materials are those which produce an image upon application of heat, pressure or both, and typically include a solid substrate and a thermosensitive layer disposed on the solid substrate. The solid substrate is often flexible, such as paper, fabric, plastic, cardboard, etc., and may itself be colored or it may comprise a support and a colored layer disposed on the support, where the support is paper, fabric, plastic, cardboard, etc. The thermosensitive layer is initially opaque and conceals the color of the underlying solid substrate, but upon application of heat, pressure, or a combination of both, to specific portions of the thermosensitive layer, those portions become transparent and reveal the color of the underlying layer to produce an image on the recording material.
The thermosensitive layer should initially be opaque when dry, and must be transparent upon application of heat to reveal the color beneath. It is preferred that both initial opacity and post-heating transparency of the thermosensitive layer be maximized. To provide a clear image, the contrast between the opaque color of unheated portions of the thermosensitive layer and the underlying color should also be maximized.
The thermosensitive layer typically contains heat deformable particles, for example thermoplastic or wax, and a binder for fixing the heat deformable particles in the thermosensitive layer upon application to the solid substrate. It may also contain additional components including, but not limited to, for example, pigments or colorants, stabilizers, lubricants, thickeners, biocides, dispersants, and emulsifiers, among others.
Japanese Unexamined Patent Application No. JP1990-080288 discloses recording sheets having a thermosensitive layer comprising polymer particles embodying internal voids as the heat deformable particles, and binders. The polymer particles having voids are sometimes referred to as hollow spherical particles (HSPs) and, in JP1990-080288, are made from a thermoplastic resin such as polystyrene, poly(alkyl) methacrylates, poly(meth)acrylonitrile, polyvinyl compounds including polyvinyl chloride, polyvinyl alcohol and polyvinyl carbazole, cellulose acetate, cellulose triacetate, and polycarbonates, among others, and copolymers thereof.
International Patent Application Publication No. WO2012/145456 also discloses recording sheets having a thermosensitive layer comprising HSPs and binders, but also contains an opacity reducer. The HSPs have core-shell architecture and the shells are formed from polymerizing one or more ethylenically unsaturated monomers, preferably methyl methacrylate, ethyl acrylate, butyl acrylate, styrene, and combinations thereof. The opacity reducer is made from organic compounds such as, for example, aromatic oxalic acid esters, aromatic ethylene glycol ethers, p-toluenesulfonamide, o-toluenesulfonamide, and waxes such as erucamide and stearic acid amide, among others.
British Patent No. 997,289 discloses heat sensitive recording material having a porous thermosensitive layer comprising synthetic thermoplastic resin particles and an adhesive (i.e., binder). The synthetic thermoplastic resin particles are the heat deformable particles and are made from polyvinyl chloride, acrylic resins, polystyrene and other similar compounds.
A thermal magnetic recording medium suitable for making debit and credit cards, transportation passes, identification cards, and the like, is described in Japanese Patent Application Publication No. JP1991-193491(A) and includes a magnetic recording layer disposed on a substrate and a thermal recording layer disposed on the magnetic recording layer. The thermal recording layer is opaque at room temperature but is rendered transparent upon application of heat and is made from a styrene/acrylic copolymer resin, which is crosslinked. It is unclear whether a binder is required.
Japanese Patent No. JP03176961B2 describes a thermal recording material also suitable for cards and passes and which includes a thermosensitive layer comprising polymer particles having a multilayered structure as the heat deformable particles. These multilayered polymer particles comprise styrene-acrylic resin and opacity is accomplished by crosslinking one of the layers which then has a different refractive index than the other layers. Upon application of heat other layers coalesce to render the thermosensitive layer transparent.
Diffuser polymer particles having a continuous refractive index gradient such that a refractive index at the center thereof (RIcenter) is different from a refractive index at the surface thereof (RIsurface) are known and sometimes referred to as gradient refractive index (GRIN) polymer particles.
For example, U.S. Pat. No. 7,893,162 B2 discloses light scattering compositions comprising single phase diffuser polymer particles having greater than 4 weight % crosslinking density and which exhibit a different refractive index at a center thereof (RIcenter) than at a surface thereof (RIsurface), and where (RIcenter)<(RIsurface). The light scattering compositions of U.S. Pat. No. 7,893,162 B2 are useful for applications such as rear projection screens, illuminated signs, light fixtures, automotive sunroofs, greenhouse glazing and other uses.
U.S. Pat. No. 7,768,602 B2 describes light diffusing articles which may, for example, be in the form of optical films useful as diffuser films for LCD televisions, flat screen computer monitors, cell phone displays, and camera screens, for example. These light diffusing articles comprise a polymeric sheet-like substrate which is optically clear, and one or more layers of a coating composition comprising solid clear polymeric lenses, which may be divergent or convergent GRIN particles, and which may be single phase or multilayer, but all of which have a refractive index which varies continuously within the particles from the center to the surface.
Improvements in the performance of thermal recording materials are still sought. For example, thermal recording materials having increased initial opacity and brightness, as well as post-heating contrast, are sought. The present invention accomplishes such improvements by including single phase GRIN particles in the thermosensitive layer along with usual heat deformable particles and binders.